iio: consumer.h: Fix kernel doc incosistency

For the iio_read_channel_raw and iio_read_channel_scale the kerneldoc comment
refers to an argument called "channel", while the argument is called "chan" in
the function signature. This leads to the following warnings from kerneldoc:

	Warning(include/linux/iio/consumer.h:71): No description found for parameter 'chan'
	Warning(include/linux/iio/consumer.h:71): Excess function parameter 'channel' description in 'iio_read_channel_raw'
	Warning(include/linux/iio/consumer.h:109): No description found for parameter 'chan'
	Warning(include/linux/iio/consumer.h:109): Excess function parameter 'channel' description in 'iio_read_channel_scale'

This patch fixes the warnings by naming them consistently.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>